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8/7/2019 2 Lecture (Refrigerants and Components)
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Introduction to P-h Charts
Refrigerants & Components
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Ideal VC cycle on P-hDiagram
Introduction to P-h Charts
Skeleton P-h Diagramfor water
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Introduction to P-h Charts
P-h Diagram for R-12
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Refrigerants
Refrigerants & Components
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The R-# numbering system was developed by DuPont and systematically identifies the
molecular structure of refrigerants made with a single halogenated hydrocarbon. The
meaning of the codes is as follows:
Numbering
Rightmost digit: Number of fluorine atoms per molecule.
Tens digit: One plus the number of hydrogen atoms per molecule.
Hundreds digit: The number of carbon atoms minus one. Omitted for methyl halides, which
have only one carbon atom.
T
housands digit: N
umber of double bonds in the molecule. This is omitted when zero, andin practice is rarely used, since most candidate compounds are unstable.
A suffix with a capital B and a number indicates the number of bromine atoms, when
present. This is rarely used.
Remaining bonds not accounted for are occupied by chlorine atoms.
A suffix ofa lower-case letter a, b, or c indicates increasingly unbalanced isomers.
Special cases: R-400 & R-500 series made up of zeotropic & azeotropic blends. R-700series is made up of natural fluids (e.g. CO
2(R744), Ammonia (R717) etc.)
e.g., R-134a has 4 fluorine atoms, 2 hydrogen atoms, 2 carbon atoms, with an empirical
formula of tetrafluoroethane. The "a" suffix indicates that the isomer is unbalanced by one
atom, giving 1,1,1,2-Tetrafluoroethane. R-134 without the "a" suffix would have a molecular
structure of1,1,2,2-Tetrafluoroethane a compound not especially effective as a refrigerant.
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Refrigerants are substances that are used to absorb and transport heat for the purpose of
cooling. Requirements for a refrigerant are as follows:
Selecting the Right Refrigerant
a. High latent heat of vaporization
b. Non-corrosive, non-toxic and non-flammable (e.g. Ammonia is toxic & flammable)
c. Critical temperature should be outside working range
d. Reasonable working pressure
e. Compatibility with component materials (e.g. Ammonia & copper reaction)
f. Stability
g. Low Cost
h. Availability
i. Environmental impact
j. Ease of leak detection
Examples:
R-12, R-134a, R-22
Propane, Ethane
Ammonia R-717, Water R-718, Carbon dioxide R-744
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Montreal Protocol (1987): CFC/halon/bromide production phase-out by 1995
Selecting the Right Refrigerant
Refrigerant emissions were about 10% of total.
Kyoto Protocol (1992): At UNCED, established legally binding commitments for the
reduction of six greenhouse gases (carbon dioxide, methane, nitrous oxide, sulfur
hexafluoride, hydrofluorocarbons, and perfluorocarbons)
Q: Each kWh of electricity used in Pakistan produces how much kg of CO2?
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Selecting the Right Refrigerant
Overall GWP =Total equivalent
warming impact
(TEWI)
CO2
emission
largest
contributor
Focus on
efficiency of Ref.
system and, thus,
refrigerant
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Compressors
Refrigerants & Components
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Air CompressorsAir Compressors Gas CompressorsGas Compressors
(gases other than air)(gases other than air)
Widely used to supplyWidely used to supply
compressed air as ancompressed air as an
energy sourceenergy source
Used in petroleum,Used in petroleum,
plastics andplastics and
chemical industrieschemical industries
Used in refrigerationUsed in refrigeration
and air conditioningand air conditioning
systemssystems
RefrigerantRefrigerant
CompressorsCompressors
General: Types and uses
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General: Types
Compressors may be of thepositive displacementordynamic type.PositivePositive
displacement compressorsdisplacement compressors trap a volume of gas in an enclosed space and increase
the pressure by reducing the volume of the space while ynamic compressorsDynamic compressorsaccelerate a gas and increase its kinetic energy & this kinetic energy is converted
into pressure.
Positive displacement compressorsPositive displacement compressors can be classified into two main
groups:
reciprocating compressorsreciprocating compressors - including trunk, crosshead, labyrinth and
diaphragm types
rotary compressorsrotary compressors - including vane, screw, liquid ring, and lobe rotor
types
Dynamic compressorsDynamic compressors include centrifugal flow and axial flow machines
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Positive Displacement Compressor: General
The general form of positive displacement compressor is the piston type, being
adaptable in size, number of cylinders, speed and method of drive. It works on the
two-stroke cycle (see Figure).
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In multicylinder compressors, four, six and eight cylinders are common for large
systems and, for small systems, it will be of two, three or four cylinders.
Piston compressors may be generally
classified by the type of valve.
Lower loads require reduction in
capacity and can be achieved in the form
of bypass ports or by speed control (when
steam driven). Cold suction gas provides sufficient
cooling for small machines. Water-cooled
cylinder heads used where high discharge
temperatures are involved.
Lubricant supply is the minimumconsistent with efficient lubrication for
cylinder lubrication whereas maximum
viscosity is consistent with efficient
lubrication forbearing lubrication.
Positive Displacement Compressors
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Refrigerator Oils
Lubrication of the compressor to minimize friction and wear
Sealing
between working parts of the compressor
Cooling
of the compressor bearings and casing
Noise reduction
of noise generated by the moving parts of thecompressor
Electrical insulation of the motor in hermetically-sealed compressors
Functions of a refrigerator oil can be as follows:
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Refrigerator Oils
What properties should refrigerator oils have?
ViscosityViscosity
must be sufficiently viscous to lubricate the compressor effectively andprovide adequate sealing where required
common refrigerant gases are extremely soluble in oil and may produce asignificant decrease in oil viscosity
MiscibilityMiscibility
in sealed systems, the oil must be completely miscible with the refrigerant
if the oil/refrigerant separates into immiscible layers the efficiency ofrefrigeration and lubrication will be impaired
there is a risk that the system will become blocked, and the compressor maybecome starved of oil
Low temperature propertiesLow temperature properties
the oil must not form waxy deposits in the cold parts of the circuit this would lead to a reduction of heat transfer and could lead to oil starvation
of the compressor
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Refrigerator Oils Thermal stabilityThermal stability
although the temperatures in refrigerator systems are not normally as high as in
air compression systems, the oil must not break down to form coke-likedeposits, e.g. on compressor chambers and valves
Chemical stabilityChemical stability
the oil must not react with the refrigerant, or act as a medium for reaction
between the refrigerant and other system components
in the presence of small amounts of air, moisture and other impurities,unsuitable oils may react with refrigerants to form sludges
CompatibilityCompatibility
the oil must be compatible with the materials used in the refrigeration system,
e.g. electrical insulation, varnishes, elastomers, polymers etc.
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Condensers
Refrigerants & Components
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Condenser: Introduction
The heat-rejection ratio (HRR) is defined as the ratio of the rate of heat rejected
at the condenser to that absorbed at the evaporator.
rate of heat rejected at condenser
rate of heat absorbed at evaporatorHRR=
The designer and operator of the refrigeration system will usually characterize plant
size by the refrigeration capacity.
Ignoring small heat gains and losses,Condenser load = Evaporator load + Compressor power
Condenser rating is stated as the rate of heat rejection. Some manufacturers give
ratings in terms of the evaporator load,
Condenser load = Evaporator load factor
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Condenser: Types
The three main types of condensers used in general refrigeration systems are:
(air-cooled) (water-cooled)
(evaporative)
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Condenser Types: Air-cooled
To provide a comparison, some of the characteristics of each are enumerated:
Air-cooled condenser: Usually lowest first cost of the three, and least maintenance cost as well, because no
water circulates or evaporates.
Example: condenser of the domestic refrigerator. Above this size, the flow of air
over the condenser surface will be by forced convection, i.e. fans.
Extended surfaces are almost always used (Ao/A
i= 5 10).
Inlet at the top and outlet at the bottom (gravity-assisted flow).
The flow of air may be vertically upwards or horizontal.
Where a single fan would be too big, multiple smaller fans give the advantages of
lower tip speed and noise, and flexibility of operation in winter.
The low specific heat capacity and high specific volume of air implies a large
volume to remove the condenser heat. In practice, the temperature rise of the air is kept between 9 and 12 K.
Materials of construction are aluminium fins on stainless steel tube for ammonia, or
aluminium orcopperfins on aluminium orcoppertube for the halocarbons.
Used in desert areas where the supply of cooling water is unreliable.
Q: On average, approx. whats the air flow rate if 430 kW is rejected by a condenser?
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Condenser Types: Water-cooled
Water-cooled condenser with cooling tower:
Lower condensing temperature than with an air-cooled condenser, because the wet-
bulb rather than the dry-bulb temperature of the air is the sink toward which thecondensing temperature drives.
The higher heat capacity and lower specific volume of water make it an ideal
medium for condenser cooling.
General form is shell-and-tube having the water in the tubes.
M
aterials can be selected for the application and refrigerant, but all mild steel iscommon for fresh water, with cupronickel or aluminium brass tubes for salt water.
Some condensers have two separate water circuits (double bundle), using the
warmed water from one circuit as reclaimed heat in another part of the system.
The supply of water is usually limited and requires the use of a cooling tower.
Optional sources may include ground water or industrial water.
Q: By comparison, whats the flow rate now for the condenser?
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Condenser Types: Evaporative
Evaporative condenser:
Compact and provides lower condensing temperatures than the air-cooled condenser
as well as the water-cooled condenser/cooling tower combination. The mass flow of water over the condenser tubes must be enough to ensure wetting
of the tube surface, and will be of the order of80160 times the quantity evaporated.
Evaporative condensers have a higher resistance to air flow than cooling towers.
Most types use forced draught fans.
Evaporative condensers may freeze in winter. A common arrangement is to switchoff the fan(s) with a thermostat, to prevent the formation of ice. The water-collection
tank will have an immersion heater or the tank may be located inside the building
under the tower structure.
Materials of construction must be corrosion resistant.
The atmospheric condenseris a simplified form of evaporative condenser, having
plain tubes over a collecting tank and relying only on natural air draught. This will belocated on an open roof or large open space to ensure a good flow of air. Space
required is of the order of 0.2 m2/kW.
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Evaporators
Refrigerants & Components
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Evaporators are classified according to their refrigerant flow pattern and their
function. The refrigerant flow pattern is usually dependent on the method of ensuring
oil removal from the evaporator.Floodedevaporators (see Figure) have a body of fluid boiling in a random manner,
the vapour leaving at the top. In the case of ammonia, any oil present will fall to the
bottom and be drawn off from the drain pot or oil drain connection. With
halocarbons, a proportion of the fluid is bled off and rectified. Evaporators which
keep the oil moving by means of continuous fluid velocity, until it gets back to the
compressor suction, are termed dry expansion. In these, the refrigerant is totally
evaporated.
Evaporators: Classification
The function of the evaporator will be to cool gas, liquid or
other product load. In most cases, air or a liquid is first
cooled, and this is then used to cool the load. For example,
in a cold-room, air is cooled and this air cools the stored
produce and carries away heat leaking through the
structure; in a water chiller system, the water is circulated
to cool the load, etc.
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Air cooling evaporators for cold-rooms, air-conditioning, etc., will have finned pipe
coils.
In all but very small coolers, there will be fans to blow the air over the coil. Construction materials will be the same as for air-cooled condensers. Aluminium
fins on copper tube are the most common for the halocarbons, with stainless steel or
aluminium tube for ammonia.
Frost or condensed water will form on the fin surface and must be drained away. To
permit this, fins will be vertical and the air flow horizontal, with a drain tray provided
under.
The size of the tube will be such that the velocity of the boiling fluid within it will
cause turbulence to promote heat transfer.
Tube diameters will vary from 9 mm to 32 mm, according to the size of coil.
Fin spacing will depend on factors like compactness and cost and will vary from 2-
12 mm. The number of air-cooling coils in operation in industrial refrigeration plants farexceeds the number of liquid-chilling evaporators installed.
Air Cooling Evaporators
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Liquid cooling is mostly in shell-and-tube or shell-and-coil evaporators.
Liquid Cooling Evaporators
In the shell-and-tube type, the liquid is usually in the pipes and the shell is some
three-quarters full of the liquid, boiling refrigerant. A number of tubes is omitted at
the top of the shell to give space for the suction gas to escape clear of the surface
without entraining liquid. Further features such as multiple outlet headers, suction
trap domes and baffles will help to avoid liquid droplets entering the main suction
pipe. Gas velocities should not exceed 3 m/s and lower figures are used by some
designers. The speed of the liquid within the tubes should be about 1 m/s or more, topromote internal turbulence for good heat transfer.
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Evaporators of this general type with dry expansion circuits will have the refrigerant
within the tubes, in order to maintain a suitable continuous velocity for oil transport,
and the liquid in the shell. These can be made as shell-and-tube, with the refrigerantconstrained to a number of passes, or may be shell-and-coil (see Figure). In both these
configurations, baffles are needed on the water side to improve the turbulence, and
the tubes may be finned on the outside.
Liquid Cooling Evaporators
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Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have
flooded or dry expansion circuitry. Flooded coils will be connected to a combined
liquid accumulator and suction separator (usually termed the surge drum), in the formof a horizontal or vertical drum.
Liquid Cooling Evaporators
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Some liquids, such as vegetable fats and ice-cream mixes, increase considerably in
viscosity as they are cooled, sticking to the heat exchanger surface. Evaporators for
this duty are arranged in the form of a hollow drum surrounded by the refrigerantand having internal rotating blades which scrape the product off as it thickens,
presenting a clean surface to the flow of product and impelling the cold paste towards
the outlet.
Liquid Cooling Evaporators
Note: There are other methodologies used as well.
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Plate evaporators are formed by cladding a tubular coil with sheet metal, welding
together two embossed plates, or from aluminium extrusions.
The extended flat face may be used for air cooling, for liquid cooling if immersed ina tank.
The major use for flat plate evaporators is to cool a solid product by conduction, the
product being in packages and held close between a pair of adjacent plates.
Has horizontal and vertical types.
Some of its popularity is attributable to its compactness.
Plate Evaporators
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Air cooling evaporators working below 0C will accumulate frost which must be
removed periodically, since it will obstruct heat transfer.
Evaporators of suitable and robust construction can be defrosted by brushing,
scraping or chipping, but these methods are labour-intensive and may lead to damage
of the plant.
Where the surrounding air is always at + 4C or higher, it will be sufficient to stop the
refrigerant for a period and allow the frost to melt off.
For lower temperatures (< 4C), heat must be applied to melt the frost within a
reasonable time and ensure that it drains away.Methods include electric resistance
heaters, hot gas and reverse cycling.
Defrosting
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Expansion Valves & Thermostatic Control Devices
Refrigerants & Components
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In most room air conditioners, a capillary tube is used as the throttling device. A
capillary tube, also sometimes called a restrictor tube, is a fixed length of small-
diameter tubing lying between the outlet of the condenser and the inlet of theevaporator. This has certain limitations, but will give reasonably effective control over
a wide range of conditions if correctly selected and applied.Mass flow is a strong
function of pressure difference. Tube bores of0.82 mm with lengths of14 m are
common. Usually made of copper.
Capillary Tube
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In a refrigeration system, the expansion valve is an adjustable throttling device through
which the refrigerant at condensing pressure is throttled to evaporating pressure or
interstage pressure. At the same time, the expansion valve regulates its opening to feedthe required amount of refrigerant to the evaporator to meet the refrigeration load at the
evaporator.
Thermostatic Expansion Valve
Thermostatic Expansion Valves (TXVs)
TXVs are the most widely used expansion devices. A thermostatic expansion valve
regulates its refrigerant flow rate to the evaporator according to the degree of superheatof vapor refrigerant leaving the evaporator. A thermostatic expansion valve consists of
a valve body, a valve spring, a diaphragm, and a sensing bulb at the outlet of the
evaporator.
When the liquid refrigerant passes through the small opening around the valve pin, its
pressure is reduced to the evaporating pressure. Liquid refrigerant flows through therefrigerant distributor and tubes, and vaporizes gradually as it flows inside the copper
tubes. At positionx, as shown in the Figure (next slide), all the liquid has been
vaporized. By the time the vapor refrigerant reaches the outlet o of the evaporator, it is
superheated to a few degrees higher than its saturated temperature.
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If load increases, more refrigerant vaporizes and degree of superheat increases which
increases temperature of sensing bulb. This exerts higher pressure on diaphram which
lowers the valve pin.More liquid refrigerant enters to match the increase in load. Thedegree of superheat of the vapor refrigerant at the outlet can be adjusted by varying the
tension of the spring in the thermostatic expansion valve.
Thermostatic Expansion Valve
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Used for dry-expansion circuits as there is no
liquid level that can be detected.
Sensing bulb usually contains the samerefrigerant.
Huntingrefers to a condition that occurs
when a device continuously undershoots or
overshoots the control point, with resulting
fluctuation and loss of control of thecondition to be maintained. Sometimes called
cycling.May reduce refrigeration capacity.
Superheat setting of the TXV at full-load
operation is between 10 and 20 F.
Thermostatic Expansion Valve
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Electric, or more truly electronic or microprocessor-controlled, expansion valves can
provide more sophisticated, effective, and energy-efficient refrigerant flow controls
than thermostatic expansion valves. Currently, three types of electric expansion valvesare widely available: step motor valves (a), pulse-width-modulated valves (b), and
analog valves (c).
Electric Expansion Valve
(a) (b) (c)
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Thermal Electric Expansion Valve
The signal from a suitable thermistor placed at the evaporator outlet will vary,
depending on whether it senses dry refrigerant gas or traces of liquid. This can be used
directly to control the current through a thermal element to modulate the expansionvalve. This device usually has no separate adjustable controller and so cannot be
incorrectly set.
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The (low-side) float valve controls the liquid refrigerant feed to maintain a constant
liquid level in the evaporator. As the liquid level in the evaporator drops, the float
ball moves downward and opens the float valve wider so that more liquid refrigerant isfed to the evaporator. In small refrigeration systems, the float chamber is often placed
directly inside the evaporator or in an accumulator, instead of a separate low-side float
chamber. Operation of the low-side float valve may be continuous or intermittent.
Float Valve